1. Technical Field
The present invention relates to an accurate in-situ growth technique based upon reflection high energy electron diffraction dynamics for accurate lattice matching of dissimilar materials.
2. Discussion of Related Art
The invention of low loss optical fibers for use as a practical optical transmission medium has stimulated tremendous growth in other areas relating to optical communications. The term "optical" as used herein refers not only to visible light but to any electromagnetic radiation which can be transmitted within dielectric fibers. The term refers to electromagnetic radiation generally of wavelength between 0.1 and 50 microns. Recently, fiber to the home systems have been proposed which would utilize surface normal electroabsorption modulators to convert downstream light into upstream dam. One such system uses a single fiber to connect two locations and effects bidirectional data transmission over this fiber using a single laser. See, e.g., T. Wood et al., Elec. Lett., Vol. 22, pgs. 346-352 (1986) and T. Wood et al., J. Light Tech., Vol. 6, pages 527-528 (1988) the disclosures of which are incorporated herein by reference. This system uses a light modulator that imprints data on the fiber return and thus avoids the need for a second laser at the subscriber location. Particularly useful for modulation are electrooptic devices whose optical properties, such as absorption or index of refraction, may be varied by application of an appropriate electrical signal. Exemplary of such electrooptical devices is the multiple quantum well (MQW). The MQW includes a plurality of layers of different semiconductor materials such as gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). Other examples of suitable materials include such III-V systems a InAs/GaAs,(In,Ga)As/GaAs,(In,Ga)As/InP and (In,Ga)As/(In,Al)As. The layers alternate between wide bandgap material and narrow bandgap material. Appropriate selection of materials, compositions and layer thicknesses permits fabrication of unique electrooptic devices.
When used as a modulator, the MQW exhibits a significant shift in the absorption edge due to the change in the confinement energy associated with the applied electric field and consequent distortion of the well. This shift in absorption is the basis for the MQW as a modulator. Since the applied field can significantly alter the light absorption properties of a properly biased MQW, light passing through the MQW will be modulated.
In the fabrication of MQW's, thin films of the semiconductor materials are grown or deposited onto substrate material in a wide variety of reactors. One such reactor is a molecular beam epitaxy (MBE) reactor. See, e.g., U.S. Pat. No. 3,751,310 to Cho. Since 1983 it has become apparent that the period of oscillatory behavior observed in the intensity of the RHEED features on initiation of growth is directly related to the growth rate. See, e.g., J. H. Neave et at., Appl. Phep. A31,1 (1983) and J. M. VanHove et al., J. Vac. Sci. and Technol. B1,741, (1983) the disclosures of which are incorporated herein by reference. In fact, monitoring of the RHEED intensity oxillations has become an established technique for the in-situ calibration of beam fluxes and the control alloy compositions for the growth of lattice matched heterostructures. The accuracy of the analysis of the RHEED oscillatory data, for a thin layer GaAs/AIGaAs material system, is about 1%. See, e.g. Turner et al., J. Vac. Sci. Technol. B8, 283 (1990). Hence, there is a real need for a technique which allows accurate lattice matching conditions to be arrived at in a quick and reproducible manner for the realization of a variety of semiconductor optical devices such as, for example, modulators, lasers and detectors.